Hot stamping system for production of a parts assembly

ABSTRACT

A hot stamping system of automotive parts to enable the production of the body in white and moving parts, which form the body of a car in a single production plant ( 1 ) from blanks. Said system consists of two by two operating groups ( 2  and  3 ) of equipment, namely: a group for the fixed parts and a group for the moving parts, each composed of production units ( 4 ), which are fed ( 5 ) with blanks, previously individually cut into the required dimensions ( 5 ) and inspected ( 6 ), for entry into the stamping tool (matrix) in accordance with the shape and dimensions of the part to be made in the respective unit, distributed ( 7 ) both to the group of fixed parts and to the group ( 3 ) of moving parts. Each of the production units comprises: a blank feed rack ( 8 ), two to three handling robots ( 9, 11  and  13 ), a Joule heating device ( 10 ), A press ( 12 ) with the tool (matrix) relative to the workpiece to be stamped, and a storage rack ( 14 ).

FIELD OF THE INVENTION

This patent relating to the manufacturing industry, in particular the automotive industry, is for a hot-stamping system of automotive parts which enables the production of a set of parts known as body in white or closures, which form the body of a car in a single production plant, thus reducing costs and manpower.

BACKGROUND OF THE INVENTION

The hot forming technique, especially in the automotive industry, has been successful in manufacturing parts with complex shapes and higher mechanical strength. This technique takes advantage of both the good formability resulting from the higher deformation temperatures reached and the transformations undergoing by the austenite during cooling of the parts. This, promoted by the use of temperable plates of smaller thickness, contributes to reducing the vehicle weight, also decreasing the fuel consumption in automobiles.

The major problem to be solved or, at least, minimized in the process, lies in the fact that the increase in the mechanical strength of the steel impairs its stampability. However, forty years of continuous evolution in metallurgy have led to advanced high strength steels (AHSS), which seek to reconcile a higher mechanical strength of the plates with minimum losses in their formability, but the unrelenting quest for improvements shows that the advances achieved so far, although spectacular, are not enough. This is because the greatly improved AHSS steel stampability is still inadequate in many practical situations. The main problem is not exactly the conformability, but rather the so-called springback that occurs after the stamping operations, and which is quite pronounced in AHSS steel plates. The sheet actually is formed but it undergoes distortion immediately after leaving the forming matrix due to residual stresses. This distortion affects, to a greater or lesser degree, the dimensional accuracy of the part.

A solution to this problem is to unlink the processing and application characteristics of the material. So, why not to heat the plates before you do their stamping? The scientific explanation for this procedure is that, under higher temperatures, the crystallographic planes of slippage of the dislocations are activated, making their migration easier.

The process of hot forming is relatively simple. First the blanks are cut from coils of low carbon steel microalloyed to boron, then introduced into a continuous furnace (with an average length of 40-60 m) for austenitization between 850 and 950° C. The steel surface is protected against the rapid oxidation that occurs under these high temperatures. In this case, the most common alternative is to coat the surface with an AlSi layer, but it is occasionally possible to do without this coating and use the protective atmosphere inside the furnace.

After a warm-up period of four to ten minutes, the blank is sent to the stamping press where it is formed and where, also, the tempering of the sheet occurs immediately inside the water-cooled matrix. After a cooling period, lasting between 15 and 25 seconds, the tempering is complete, occasional burrs on the part are removed and it is extracted from the matrix at a temperature of about 150° C. The subsequent cooling of the part is done in contact with the air, with a tempering effect.

Hot stamping is not an exactly novel process, as it was originally developed in Sweden in 1973, just in the year in which the first major challenges for the auto industry arose because of rampant increases in oil prices. However, its industrial application was limited until the beginning of the new millennium, when its acceptance increased exponentially. Currently hot stamping is mainly used for the production of anti-intrusion components and structural reinforcements for passenger cars, whose characteristics such as a relatively large size, complex shape and high levels of mechanical strength make it difficult to use cold stamped AHSS steels.

It is estimated that in 2004 this process consumed from 60,000 to 80,000 tons of plates in Europe alone. In 2008, just before the crisis, it was anticipated that the process would consume 300,000 tons of sheets just in Europe the next year, and that another 20 new hot stamping lines would be deployed in Europe.

The hot stamping process was also successful in the USA: in 2004, 33 hot-stamped bumper versions were compiled in the country for various models at several automobile assemblers, such as Volkswagen, General Motors, Citröen, Ford and Smart. The main steels used were SAE 10B21 and 15B21, in the form of blanks with thicknesses between 1.50 and 3.50 mm. The average values of the typical post hardening properties were: 1,140 MPa yield limit; 1,520 MPa resistance limit; 12% total elongation and 479 HV hardness. Currently 22Mn B5 steel is widely used.

In hot stamping, the most critical step in defining the mechanical properties of the finished part is quenching, which is usually done within the water-cooled matrix. Therefore, the chemical composition of the steel to be used is defined according to the final mechanical properties required, the thickness of the workpiece and the cooling rates obtained during quenching in the matrix, guaranteeing an adequate level of temperability.

The best alloy designs for hot stamping steels have a carbon content of about 0.20% and relatively high manganese contents. Although the cold forming limits of AHSS steels have not yet been properly achieved, currently the hot stamping is the only process way that allows manufacturing components with complex shapes and high mechanical strength. The steel austenitization, besides facilitating the formation process due to the lower mechanical resistance and greater ductility of the material under processing, also allows to take advantage of the phenomena resulting from the further metallurgical transformation to obtain profiles with mechanical properties that allow to reduce the weight of the parts and to increase their performance.

However, the use of large furnaces (with an average length of 30 to 60 m) and the high temperatures reached make process and forming equipment more complex, which implies great investments, high energy consumption to heat the part and loss of productivity caused by the time spent in the heat treatment steps.

BRIEF DESCRIPTION OF THE INVENTION

This invention seeks to improve the production of stamped parts used in the automotive industry, features a hot stamping system that allows to stamp the whole set of parts (“body in white”, “car body” or “closures”) that forms the body of a car in a single industrial plant. It is a production system that uses a hot forming process and specific Joule heating equipment (whose patent applications were filed by the same inventor at this institute), a stamping system that aims at reducing the weight of vehicles, optimizing energy efficiency, and increasing vehicle safety due to the possibility of using steels with high mechanical strength.

This system replaces the conventional furnace by the heating equipment, promoting a considerable reduction in the spatial occupation ratio of the conventional installation area (from 1,800 m² to 300 m²). As a result, the costly maintenance of the furnace is eliminated, making possible the intermittent stamping of parts without the need of soaking, a high costs factor in the use of conventional furnaces, especially in continuous production, because it is necessary to use high soaking temperatures; therefore, energy consumption and equipment deterioration in heating the furnace atmosphere are greater.

This system further allows the use of steels with hot and electrolytic galvanizing metal coatings (Zn, ZnFe, ZnNi, among others produced in their respective coating processes) through the process mentioned above, whereas in the conventional process steel with AlSi metallic layer coatings are used. Furthermore, the conventional process produces a superficial layer in the parts that allows the presence of micro cracks, which are minimized in the process used in the system presented.

The hot stamping system for producing a set of stamped parts, subject of this patent, can be better understood by referring to the attached figures, which are included in this descriptive report and contain numerical references together with the description of their technical details. These figures do not restrict their configuration as to their dimensions, proportions and possible types of finishing inserted nor to the scope of their practical application.

BRIEF DESCRIPTION TO THE DRAWINGS

FIG. 1 is a flowchart of the production process, showing the operating phases of production.

FIG. 2 is an illustration of the production unit showing the equipment involved in the stamping.

DETAILED DESCRIPTION OF THE INVENTION

According to these figures and their numerical references the object of this patent relates to a hot forming system for the production of the set of fixed and movable parts (“body in white”, “body car” or “closures”) that form the body of the automobile, stamped in a single production unit (1). This system consists of two groups (2 and 3) of equipment, one group (2) for the fixed parts and another group (3) for the moving parts. Said groups (2 and 3) each are composed by production units (4) (as many as necessary for production of the assembly) which are supplied (5) with blanks, previously cut individually into the required dimensions (5), after being inspected (6), rendered in conditions of entry into the stamping tool (matrix) in accordance with the shape and dimensions of the part to be made in the respective production unit (4), and are distributed (7) both to the group (2) of fixed parts and to the group (3) of moving parts.

In the groups 2 and 3, the production units (4), one for each part specification to be produced, are composed of a blank feed rack (8), two handling robots (9 and 11) and a stamping unit formed by a Joule heating device (10) and a press (12) with the tool (matrix) relative to the part to be stamped. In them, the first robot (9) feeds the heating device (10) by withdrawing the blank from the feed rack (8) and arranging it in the heating device (10). After the plate has been heated, this robot (9) removes the heated part and feeds the tool on the press (12) for form and shape as determined. Thereafter the second robot (11) removes the shaped part from the press (12) and places it on a storage rack (13).

The movement of both hot and cold parts can be performed both by robotic arms and by dedicated conveying transfer systems designed specifically for the applications and, in the case of hot part moving, constructed of material that supports temperature and radiation from the plate, without being damaged.

In the heating device 10, the heating takes place by the electric conduction process (Joule heating), which can reach heating rates of up to 400° C./sec. Its arrangement close to the press (12) occupies an area slightly larger than the part to be heated, thus obtaining a greater utilization of the area available for each of the production units (4). 

1) A hot stamping system for production of a parts assembly, characterized in that it allows the hot forming of the set of fixed and movable parts (“body in white”, “body car” or “closures”, which form the automobile body, in a single industrial plant (1); said system consisting of two groups (2 and 3) of operating equipment, one group (2) for the fixed parts and another group (3) for the moving parts; said groups (2 and 3) each composed by production units (4) (as many as necessary for production of the assembly) which are supplied (5) with blanks, previously cut individually into the required dimensions (5); after being inspected (6), rendered in conditions of entry into the stamping tool (matrix) in accordance with the shape and dimensions of the part to be made in the respective production unit (4), distributed (7) both to the group (2) of fixed parts and to the group (3) of moving parts. 2) The hot stamping system for production of a parts assembly according to claim 1, characterized in that in the groups (2 and 3) the production units (4) are each assigned for stamping of a type of part, according to its specifications and determined form. 3) The hot stamping system for production of a parts assembly according to claim 1, characterized in that each of the production units (4) is composed of a blanks feeding rack (8), two handling robots (9 and 11) and a stamping unit formed by a Joule heating device (10) and a press (12) with the tool (matrix) relative to the part to be stamped. 4) The hot stamping system for production of a parts assembly according to claim 1, characterized in that in each production unit (4), after the first robot (9) has taken the blank off the feed rack (8) to arrange it in the heating device (10) and then, once the plate has been heated, removes the heated part and feeds the tool on the forming and cooling press (12) through icy water; once the part has been formed, the second robot (11) withdraws the shaped part from the press (12) and places it on a storage rack (13). 5) The hot stamping system for production of a parts assembly according to claim 1, characterized in that the movement of the parts, both hot and cold, can be carried out by both robotic arms and dedicated conveying transfer systems, designed specifically for the applications; and, in the case of hot part moving, is constructed of material that supports temperature and radiation from the plate, without being damaged. 6) The hot stamping system for production of a parts assembly according to claim 1, characterized in that the heating equipment (10) is arranged next to the press (12) occupying an area slightly larger than the workpiece to be heated. 7) The hot stamping system for production of a parts assembly according to claim 1, characterized in that the heating device (10) heats the plate by Joule effect; it may reach heating rates up to 400° C./sec. 